1. Field of the Invention
This invention relates generally to thermal heads which are widely used in facsimile machines, word processors and so forth. More specifically, the invention relates to a thermal head of the type which comprises a head circuit board carrying a heating resistor line divisionally actuated by at least on drive IC.
2. Description of the Prior Art
In general, a thermal head comprises an elongate head circuit board which carries a heating resistor line extending along one longitudinal edge of the circuit board. The circuit board also carries an array of drive IC's for divisionally actuating the resistor line to provide a line of multiple heating dots, and a printed conductor pattern which makes a sophisticated circuit arrangement on the circuit board.
In such a thermal head, because the drive IC's are relatively sensitive to shocks and mechanical forces, it is proposed to protect the drive IC array by a metallic cover member, as disclosed for example in Japanese Utility Model Application Laid-open No. 57-121543 (Laid-open: Jul. 28, 1982). Obviously, the cover member prevents the drive IC array from coming into contact with an external objects.
Further, the Japanese utility model document also proposes that the metallic cover member is grounded. During operation of the thermal head for printing, an ink ribbon and/or paper (including thermosensitive paper) may be electrostatically charged due to inevitable sliding movement thereof relative to the heating resistor and/or the platen, and the electrostatic charge of the ribbon and/or paper induces opposite electrostatic charge on the cover member. If this happens, the grounded cover member allows electrostatic charge to escape from the cover member, thereby preventing the drive IC's from being electrostatically disturbed or damaged.
However, the thermal head disclosed in the above Japanese utility model document is disadvantageous in that the metallic cover member increases the size and weight of the thermal head.
For realization of size and weight reduction, it has been recently proposed to enclose the array of drive IC's and their respective wires in a resinous package which is made of a relatively hard resin such as epoxy resin, unsaturated polyester resin or polyetheramide resin. The use of the resinous package makes it unnecessary to use any protective cover member.
However, the use of the resinous package is defective in that it cannot eliminate the problem of electrostatic influences, as opposed to the metallic cover member disclosed in the Japanese utility model document. Specifically, when the package is made of epoxy resin, unsaturated polyester resin or polyetheramide resin, the volume resistivity of the package is approximately from 10.sup.13 to 10.sup.14 (.OMEGA..multidot.cm) or even higher. Due to such a high volume resistivity or high electrical insulation, the resinous package is easily charged electrostatically at a high voltage with a small electrostatic capacity. As a result, the drive IC's becomes liable to malfunction, and the heating resistor may be damaged by discharging upon passage of the oppositely charged ink ribbon or paper.